|MadSci Network: Physics|
A. We must observe that if all the water in the lake would freeze there could be no living superior organism (like a fish) in there. B. The main reasons why only a thin cover neighbouring the contact surface between water and air freezes are: - an odd property of water: it has a lower density at a temperature of 4 degrees Celsius than it has at 0 degrees Celsius. - the heat exchange between water and air is faster than the water-ice or water-earth heat exchange. C. Now let's look in more detail at this two reasons. 1. As you probably know, bodies tend to dilate (raise their volume) when heated. (E.g.: The railways of old trains or of the tram in San Fransisco are made of pieces with a little space left between them. This is why you hear that periodical noise when using one of thease. When the air has a temperature at 2 meters above earth surface of - let's say - 45 degrees C, the railways (metal parts of ...) will reach 60 degrees C. Because of this the individual pieces tend to become longer and, sometimes, if the space left between them at normal temperature is insufficient then they might change their form so that a train will no longer advance in a rectilinear motion.) So now we know from experience that bodies tend to raise their volume if temperature raises. Of course, the mass of a dilating body remains constant because there is no change in the amount of substance it contains. This means we can say that a body which has a higher temerature tends to have a lower density (=mass/volume). Another thing to point at is that in a liquid bodies with lower desities will go upwards, while those with higher densities (compared to that of the liquid) tend to go downwards. If you want more information about this search for Archimedes law. (E.g.: A rock - which has a higher density compared to that of water - will go down and settle at the bottom; a piece of wood - which has a lower density - will go up and settle at the surface, that is - it floats.) Now that we know all this we might assume that if we have water of two different temperatures in a tank then the one with a higher temperature (which means lower density) should stay on top of the other one. But this is true only for temperature greater than 4 degrees C. At temperatures between 0 and 4 degrees C water has the peculiar property of having a greater density for greater tempertures. In other words, it dilates while _cooling_. This is why if we have water of two different temperatures - one 0 degrees C and the other 4 degrees C - the one which has a temperature of 4 degrees will stay below the water with temperature 0 degrees C (before energy exchange as heat begins to have a considerable effect). 2. Now let's see how the heat exchange takes place between water and air and then between water, ice and earth. When one wants to dry some laundry a good method is to hang them on a wire outside (if it is warm). After a time we observe that they cool and dry. This is due to evaporation (transformation of water from liquid into gas-vapours). But if you put outside a brick it will not cool, it will even get warm. But why a does a substance cools when it evaporates? First we must explain what is temperature. We know that little tiny microscopic parts of objects (atoms / molecules) are always moving. We call this a thermic movement. (the macroscopic effect of this is the brownian motion) If this little parts move generaly faster than others in another body we say that the first body has a higher temperature than the other one. At the contact surface between water and air something interesting happens. Some of the faster molecules in the liquid escape. If we have less faster molecules we can say that the ones which remains move generaly slower so they have a _lower_ _temperature_. Another thing: The heat exchage is faster if the difference in temperature between the bodies is higher. But below ice (in our lake) is water with temperature 0 (and below this it gradualy reaches 4 degrees). So ice acts as an heat insulator. The flow of heat to the earth is somehow higher than the one to the ice but it is still small.
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